<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box{
      height: 200px;
      width: 200px;
      background-color: green;
      /* 盒子消失 */
      /* display: none; */
      opacity: 0; /* transition 过渡效果 */
      /* overflow: hidden; */
      /* visibility: hidden; */
      /* position: absolute; */
      /* position: absolute;
      left: -999px; */
      /* transform: scale(0);  缩放 */

    }
    .box:hover{
      opacity: 1;
      transition: opacity 5s;
    }
    /* 父容器  实现*/
    /* .container{
      height: 0;
      overflow: hidden;
    } */
  </style>
</head>
<body>
  <div class="box"></div>
</body>
</html>